Furnace installation for burning liquid or gaseous fuel, in particular for a boiler

ABSTRACT

A furnace installation for burning liquid or gaseous fuel with recycling of the flue gases, in particular for a boiler, comprising at least one burner directed through a flame hole in the wall of the combustion chamber of the installation, and a duct member for conducting secondary combustion air to the flame hole, the recycled flue gas being supplied to the flame hole through openings arranged circumferentially in the duct member adjacent the flame hole whereby the flue gas flowing through these openings and through the flame hole into the combustion chamber forms a cooling mantle of flue gas enveloping the flame of the burning fuel emanating from the burner.

[ FURNACE INSTALLATION FOR BURNING LIQUID OR GASEOUS FUEL, IN PARTICULAR FOR A BOILER [75] Inventor: Robert Cornelis Schol, Hilversum,

Netherlands [73] Assignee: lngenieursbureau Rodenhuis &

Verloop N.V., Hilversum, Netherlands 22 Filed: Sept. 1,1972

21 Appl. No.: 285,856

[30] Foreign Application Priority Data Jan 6, 1972 Netherlands 7200207 [52] US. Cl 110/49, 431/D1Ci. 13, 122/479 A, 432/222 [51] Int. Cl F23j 5/02 [58] Field of Search 110/49, 22, 23, 51, 52; 432/222; 431/202, 278, 280, DIG. 13; 122/479 A [56] References Cited UNITED STATES PATENTS 1,729,763 10/1929 DeFlorez 43l/D1G. 13

[ Oct. 1, 1974 2,224,544 12/1940 Keller 122/479 A 2,701,608 2/1955 Johnson llO/49 2,993,479

7/1961 Thurley 43l/D1G. 13

Primary Examiner-John J Camby Assistant ExaminerHenry C. Yuen Attorney, Agent, or 'FirmEric H. Waters 57 ABSTRACT A furnace installation for burning liquid or gaseous fuel with recycling of the flue gases, in particular for a boiler, comprising at least one burner directed through a flame hole in the wall of the combustion chamber of the installation, and a duct member for conducting secondary combustion air to the flame hole, the recycled flue gas being supplied to the flame hole through openings arranged circumferentially in the duct member adjacent the flame hole whereby the flue gas flowing through these openings and through the flame hole into the combustion chamber forms a cooling mantle of flue gas enveloping the flame of the burning fuel emanating from the burner.

I 8 Claims, 3 Drawing Figures Pmm num H974 sum so; 5%

FURNACE INSTALLATIONFOR BURNINGLIQUID OR GASEOUS FUEL, IN PARTICULAR FOR A BOILER BACKGROUND OF THE INVENTION The invention relates to a furnace installation for burning liquid or gaseous fuel with recycling of the flue gases, in particular for boilers. More in particular, the invention relates to such a furnace installation having a furnace wall enclosing a combustion chamber and provided with at least one flame hole, a burner being mounted .in line with this flame hole for directing fuel through the hole to be burned in the combustion chamher, which flame hole is in communication with a pressure source for secondary combustion air whereby this air flows past the burner and through the flame hole into the combustion chamber while mixing with the fuel injected by the burner.

In the operation of such furnace installations high flame temperatures may occur and it is known that such high flame temperatures result in the forming of oxides of nitrogen which are harmful to the environment. Attempts have therefore been made to reduce the normal temperature for oil and natural gas flames of approximately l,700-l ,800 C to below the borderline temperature of approximately 1,300 C which is critical to the development of such nitrogen oxides.

It is further known to control the flame temperature of such furnace installations by leading an adjustable part of the consumed flue gases together with the secondary combustion air to the burners in dependence on the boiler load in order that, as the load decreases, the speed of the gas flowing through the superheater can be maintained at substantially the same level so as to keep the performance of the superheater and the temperature of the produced superheated steam as constant as possible. It has also been suggested to add a specific amount of flue gas to the secondary combustion air at full load in order to achieve a lower flame temperature. In the arrangement of this type known thus far the flue gas is conducted directly into the wind box for the combustion air. This suffers from the disadvantage, however, that this combustion air and the flue gas added to it generally have different temperatures and therefore do not mix well which leads toan irregular flame and makes it difficult to achieve good control.

SUMMARY OF THE INVENTION The invention has for its main object to provide a furnace installation of the kine above referred t which obviates the above-discussed disadvantage of the known arrangements and which allows to keep the flame temperature below the limit which is critical to the development of nitrogen oxides while maintaining a stable flame and a good control both at full load and at reduced load.

In the furnace installation according to the invention, the recycled flue gas is supplied to each one of the flame holes by conducting means which open circumferentially of the flow of secondary combustion air adjacent the periphery of the flame hole whereby the flue gas flowing together with this combustion air through the flame hole forms a mantle of flue gas enveloping and thereby cooling the flame of the burning fuel injected by the burner.

The flame is thus cooled in the desired manner directly it is formed by means of the flue gas mantle enveloping it without the necessity of first mixing the flue gases with the combustion air. This way of flame cooling also offers a further considerable advantage in terms of control technique. On the basis of a boiler design with, for example, 15 percent flue gas feed into the combustion chamber at full load, this proportion is increased as the load falls, so that, as is described above, the mass flow through the superheater is kept at a reasonable value. In the arrangement of the invention, an increase in the amount of flue gas supplied has the effect of causing the neck of the burner assembly to be-. come seemingly narrower and consequently, when a smaller amount of combustion 'air is supplied, the fall in pressure in the neck and thus the speed of the combustion air flowing outwards changes only slightly. This means that over a wide range of load adjustment a substantially constant mixing energy per unit of combustion air volume supplied and thereby optimum mixing of the fuel with the combustion air are obtainable.

In an installation having a plurality of bumers it is preferable to supply the recycled flue gas to each burner through an individually adjustable flue-gas control valve for this burner. The advantage of this is that by means of these control valves the mixture ratio between the combustion air and the flue gas can be individually and differently adjusted for every burner. It is thus possible to have a first burner group work in super-stoichiometric state and to have a second group work in sub-stoichiometric state so that exactly the desired amount of combustion air enters the combustion chamber for the flames of both burner groups intoto. As there is a drop in the flame temperature in both mentioned burner states, caused by cooling of the flame as a result of a surplus of cool combustion air and as a result of initially incomplete combustion caused by an inadequate amount of air, respectively, a cooler flame is thus achieved on all burners without loss of performance.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the invention will appear from the following description, in conjunction with the accompanying drawings, of a preferred embodiment of the furnace installation according to the invention. In the drawings:

FIG. 1 is an axial section through one of the burner locations of the installation;

FIG. 2 is a view of the flue gas box of the installation in a section along theline IIII of FIG. 1; and

FIG. 3 is an axial section through the burner head on an enlarged scale.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION The drawings show one burner location of a boiler fitted, in a known manner, with a plurality of burners. The boiler has a combustion chamber 1 within a furnace wall 2 on which are mounted boiler tubes 3 and a boiler wall insulation 4, as is shown diagrammatically in FIG. 1. The furnace wall has a number of flame holes such as 5 each with a similar burner 6 mounted there behind, only one of which can be seen in the drawings.

The burner 6 can be of any suitable type for burning liquid and/or gaseous fuels. The burner illustrated here is of the kind as described in my British Pat. No. 1,080,528, being a burner for fuel oil as well as for natural gas. The burner has a double-walled cylindrical casing with an outer wall 7 and an inner wall 8 which enclose a ring-shaped gas channel 9 into which natural gas or another suitable gaseous fuel can be supplied by means of a gas supply conduit 10 which is attached to the rear end of the outer wall 7 and extends rearwards. Both walls 7 and 8 run conically inwards at the front end and meet up with a conical front wall 11 which seals off the space 9 and in which is provided an annular row of outlet bores 12 for the fuel gas. A coneshaped flame funnel 13 which extends forward isattached to this front wall 11. Immediately behind this funnel 13 a crown of outlet tubes 14 for the fuel gas are screwed into the in-turned end of the outer wall 7, their axes lying on a conical surface and running parallel to the generatrices of the funnel 13. Behind the funnel 13 there is an air deflecting bushing 15 attached to the outer wall 7 by means not shown, this bushing widening conically at its front side to match the shape of the funnel l3 and becoming likewise conically wider at the rear. Between the outer wall 7 with the flame funnel 13 attached thereto and the air deflecting bushing 15 an annular air channel 16 which has a contracted throat at the middle, is thus formed, so that mixing air is conducted, in a way which will be described below, along the outflow tubes 14 for the gas.

An oil burner 17 is fitted into the gas burner casing 7, 8 and can again be of any suitable type, but in the example shown it is designed in accordance with my Dutch Pat. Application 270,257 and provides twostage atomization of the oil by means of primary com bustion air or atomizing air under higher pressure than the secondary combustion air. The oil is supplied through an oil feed conduit 18 which runs centrally through the casing 7, 8 and is attached at its rear end to a flexible feed pipe 21 via a coupling member 12 which is fitted with an oil pressure gauge 19. The atomizing air is led in within the inner casing wall 8 which has at its rear end a lateral connecting member 22 for connecting with a source of compressed air which supplies air under constant pressure.

The combined gas and oil burner 6 lies with its burner mouth in the centre directly behind the circular flame hole 55 and extends rearwards through a wind box 23 which is common to the various burners. This wind box 23 lies enclosed between an inner wall 24 and an outer wall 25 and by means of natural draught or by means of a forced draught fan supplies the secondary combustion air. The outer wall 25 of the wind box has an aperture 26 which is sealed off by a burner front plate 27. which is removably attached. This front plate 27 supports the outer casing wall 7 of the burner by means of a bushing 28 and reinforcing plates 29. The gas feed conduit 10, the oil feed conduit 18, and the inner casing wall 8 with the connecting member 22 for the atomizing air, extend sealingly through this front plate 27 and a dome-shaped member 30 attached to it.

A rectangular casing 31, through which the burner 6 extends, is attached to the inner wall 4 of the wind box 23, this casing having pivotable shutter flaps 32 connected to each other and mounted in two oppositefacing casing walls. The flaps 32 are adjustable by means of an operating rod 33, which extends through the wall 25 of the wind box, and an operating mechanism 34 attached to this wall, so as to control the supply of secondary combustion air to the burner in a known manner.

The above-described parts of the furnace installation are as such of known construction and design.

According to the invention, a flue-gas box 35 is built against the furnace wall 2 of the boiler and around the flame hole 5 in this wall. The box 35 has an approximately elliptical peripheral shape (FIG. 2) with circumferential side wall 36, extending between the inner wall 24 of the wind box 23 and the furnace wall 2, and with a back wall 37 which fits into a corresponding aperture in the wall 24 of the wind box. A tubular sleeve 38, coaxial with the burner, encloses the head of the burner and is housed in the flue-gas box 35, which sleeve has a smaller diameter than the flame hole 5 and is connected on the one hand by a funnel-shaped wall portion 39 to the inner edge of the flame hole 5 and on the other hand by a funnel-shaped wall portion 40 to the edge of an aperture 41 in the back wall 37 of the fluegas box, said aperture 41 having approximately the same diameter as the flame hole 5. In this way there is formed between the wind box 23 and the flame hole 5 a throughflow duct or channel for the secondary combustion air, which encloses the foremost part of the burner 6, and has a constriction 42 around the head of the burner.

The forward part of the sleeve 38 has a large number of outflow apertures 43 for the flue gas which are distributed in series circumferentially of the sleeve and run obliquely inwards and forwards as shown in FIG. 3.

A circular aperture 44, in the back wall 37 of the flue gas box 35 next to the aperture 41, receives a tube 45 connected by means of an expansible member 46 to a tubular conduit 47 which extends through the wall 25 of the wind box 23 and housesa butterfly valve 48. The tube 45 and conduit 47 form a flue-gas duct fed by a flue-gas collecting channel (not shown) from which the flue-gas ducts of the various burner locations branch off and to which the part of the boiler flue gas which is to be recycled is supplied by known means. With the help of the valve 48 it is thus possible to regulate the amount of flue gas supplied to each burner.

When using the burner 6 as a gas burner natural gas is, for instance, supplied to the feed conduit 10, while the oil conduit 21 is closed. The secondary combustion air from the wind box 23 flows around the burner through the constriction 42 into the combustion chamber. Part of this air moving along the outer wall 7 of the burner casing flows through the annular channel 16 within the deflector bushing 15 to provide mixing air for the gas emerging from the gas outlet tubes 14 whereupon the resultant air-gas mixture is mixed again with the secondary combustion air flowing through the constriction 42 so that the result is a ring-shaped flame. Gas also enters the space within the flame funnel through the narrow gas outflow bores 12, and is thoroughly mixed with the primary air emerging with pronounced turbulence from the mouth of the coil burner 17, the result being a constant stable fan-shaped flame at the mouth of the burner 17 which serves as a supporting flame for the ring-shaped main flame. If the burner 6 is used as an oil burner the gas supply is closed and oil is supplied under pressure to the oil feed conduit 18. The oil which emerges is atomized in two stages, in the manner as described in the above mentioned Dutch Pat. Application 270,257, by means of the emergent atomizing air which has been subjected to pronounced turbulence, and is then mixed with the secondary combustion air flowing through the constriction 42. In specific cases it is also possible to burn gas and oil simultaneously in the burner 6. v

The pressure of the flue gas in the flue-gas box 35 is somewhat higher than that of the air in the wind box 23. During the operation of the burner, the flue gas which flows through the apertures 43 into the constriction 42 forms a mantle 49 of flue gas whose boundary lines 49 are shown diagrammatically in FIG. 1 and which gradually spreads over some distance in the form of a funnel in the combustion chamber 1 from the perforated part of the sleeve 38 and the funnel-shaped wall portion 39. This mantle 49 of flue gas thus forms an evelope for the burning air/fuel mixture which emerges from the burner 6, its outline being'shown diagrammatically by 50. This flame 50 is consequently cooled in its hottest part by the flue-gas mantle 49 enveloping it whereby the flame temperature can be brought below the above-mentioned critical boundary temperature of l,300 C.

As can be seen in the drawings, the emerging mantle of flue gas initially bends inwards from the outflow apertures 43 and has its smallest internal diameter (smaller than the diameter of the constriction 42) at the point indicated by the broken line 5] just in front of the mouth of the flame hole 5. The minimum diameter of this mantle throat 51 depends upon the amount of flue gas which is flowing out, and this can be adjusted by means of the valve 48. When the boiler is under full load, in which case, for instance, -20 percent flue gas is added relative to the total volume of out-flowing gas, this minimum diameter will be only slightly smaller than that of the constriction,42. With a smaller boiler load appropriate adjustment of the valves 32 and 48 makes the proportion of flue gas greater, however, whereby the mantle throat 51 becomes smaller. As has already been explained above, the result of this contraction is that the pressure head for the combustion air which is now flowing out in a smaller quantity, and thus also the speed of this air, remain approximately the same, which fact greatly contributes to the achievement that, also at a smaller load, a proper mixing of fuel and air is maintained.

As has already been described, the flue gas control valves 48 can be individually adjusted for the different burners so that one burner group can work in a superstoichiometric state and one burner group in a substoichiometric state, which provides the abovementioned favourable additional possibilities for lowering the flame temperature.

What is claimed is:

l. A furnace installation for burning liquid or gaseous fuel with recycling of the flue gases, in particular for a boiler, comprising a combustion chamber; a furnace wall enclosing said combustion chamber and having at least one flame hole; a burner mounted in line with said flame hole for directing fuel through said hole to be burned in said combustion chamber; a source of secondary combustion air; duct means for conducting said secondary combustion air to said flame hole; means for conducting recycled flue gas to said flame hole. said means opening circumferentially into said duct means adjacent to said flame hole whereby said flue gas-flowing through said flame hole forms a mantle of flue gas enveloping the flame of said burning fuel; a flue gas box mounted on the outside of said furnace wall around the flame hole, said box having a back wall opposite said furnace wall and provided with an aperture in line with said flame hole; a flue gas conduit opening into said box; sleeve means extending through said box and coaxially enclosing said burner, said sleeve means having a forward end joining the periphery of said flame hole and having a rear end attached to the edge of the aperture in said box rear wall whereby a ring-shaped duct for said secondary combustion air is formed between said sleeve and said burner and passing through said flue gas box and communicating with said source of secondary combustion air, said sleeve having outflow apertures for the flue gas uniformally distributed around its circumference and opening into said duct.

2. The installation of claim 1 including an adjustable flue gas valve mounted in said flue gas conduit.

3. The installation of claim 1, in which said outflow apertures for said flue gas, as seen in the direction of flow, run obliquely forwards and inwards from 'said flue gas box into said annular duct.

4. The installation of claim 2 in which said sleeve means comprises a cylindrical sleeve section of smaller diameter than said flame hole and said aperture in said flue gas box back wall, a first conical sleeve part connecting said cylindrical sleeve section to the periphery of said flame hole, and a second conical sleeve part connecting said cylindrical sleeve section to said aperture in said flue box back wall.

5. The installation of claim 4 in which said flue gas outflow apertures are in said cylindrical sleeve section adjacent said first conical sleeve part.

6. The installation of claim 1 further comprising a wind box for said secondary combustion air, the burner being housed in said wind box, said wind box joining said back wall of said flue gas box and communicating with said aperture in said back wall, and a control valve means arranged in said wind box controlling the flow of secondary combustion air from said wind box to said aperture.

7. The installation of claim 6 wherein said flue gas conduit passes through said wind box to said flue gas box.

8. The installation of claim 1 including a plurality of burners, said flue gas conducting means comprising a flue gas supply conduit for each of said burners and an individually adjustable flue gas valve in each of said supply conduits. 

1. A furnace installation for burning liquid or gaseous fuel with recycling of the flue gases, in particular for a boiler, comprising a combustion chamber; a furnace wall enclosing said combustion chamber and having at least one flame hole; a burner mounted in line with said flame hole for directing fuel through said hole to be burned in said combustion chamber; a source of secondary combustion air; duct means for conducting said secondary combustion air to said flame hole; means for conducting recycled flue gas to said flame hole, said means opening circumferentially into said duct means adjacent to said flame hole whereby said flue gas flowing through said flame hole forms a mantle of flue gas enveloping the flame of said burning fuel; a flue gas box mounted on the outside of said furnace wall around the flame hole, said box having a back wall opposite said furnace wall and provided with an aperture in line with said flame hole; a flue gas conduit opening into said box; sleeve means extending through said box and coaxially enclosing said burner, said sleeve means having a forward end joining the periphery of said flame hole and having a rear end attached to the edge of the aperture in said box rear wall whereby a ring-shaped duct for said secondary combustion air is formed between said sleeve and said burner and passing through said flue gas box and communicating with said source of secondary combustion air, said sleeve having outflow apertures for the flue gas uniformally distributed around its circumference and opening into said duct.
 2. The installation of claim 1 including an adjustable flue gas valve mounted in said flue gas conduit.
 3. The installation of claim 1, in which said outflow apertures for said flue gas, as seen in the direction of flow, run obliquely forwards and inwards from said flue gas box into said annular duct.
 4. The installation of claim 2 in which said sleeve means comprises a cylindrical sleeve section of smaller diameter than said flame hole and said aperture in said flue gas box back wall, a first conical sleeve part connecting said cylindrical sleeve section to the periphery of said flame hole, and a second conical sleeve part connecting said cylindrical sleeve section to said aperture in said flue box back wall.
 5. The installation of claim 4 in which said flue gas outflow apertures are in said cylindrical sleeve section adjacent said first conical sleeve part.
 6. The installation of claim 1 further comprising a wind box for said secondary combustIon air, the burner being housed in said wind box, said wind box joining said back wall of said flue gas box and communicating with said aperture in said back wall, and a control valve means arranged in said wind box controlling the flow of secondary combustion air from said wind box to said aperture.
 7. The installation of claim 6 wherein said flue gas conduit passes through said wind box to said flue gas box.
 8. The installation of claim 1 including a plurality of burners, said flue gas conducting means comprising a flue gas supply conduit for each of said burners and an individually adjustable flue gas valve in each of said supply conduits. 